The present invention relates to a distributor type fuel injection pump for distributing and supplying pressurized fuel to combustion chambers of an internal combustion engine and, more particularly, to an inner cam distributor type fuel injection pump, including a rotor which is rotatably driven, plungers disposed in the rotor to be slidable along the radial direction thereof, and a cam ring which is positioned to surround the rotor, and an inner surface of which is formed to be a cam surface to reciprocate the plungers along with rotation of the rotor, wherein fuel is drawn by reciprocation of the plungers in a pump chamber defined between the plungers, the fuel is pressurized, and then the pressurized fuel is distributed and supplied to each cylinder of the engine.
As a distributor type fuel injection pump of this type, the above-mentioned inner cam type fuel injection pump and a face cam type fuel injection pump are known. The face cam type fuel injection pump includes a plunger which is reciprocated to perform a pump action, and the plunger is reciprocated by cooperation of a face cam, which is rotated with the plunger, and cam rollers rotatably contacting the face cam. However, in this face cam type fuel injection pump, although the face cam is urged against the cam rollers by force of a spring, a jump phenomenon of the face cam, i.e., separation of a cam surface of the face cam from the cam rollers, occurs when rotational frequency of the rotor, and hence, the face cam, increases. Such a jump phenomenon of the face cam disturbs reciprocation of the plunger, i.e., the pump action of fuel. For this reason, it is difficult to inject fuel at high speed or to increase a rate of injection in the face cam type fuel injection pump. In addition, since the cam surface of the face cam is lubricated by the fuel itself, durability of the cam surface of the face cam is degraded if the fuel is of poor quality.
Unlike the face cam type fuel injection pump described above, since a jump phenomenon of the cam ring is almost never found in the inner cam type fuel injection pump, the inner cam type fuel injection pump is superior to the face cam type fuel injection pump in terms of high speed fuel injection and the rate of injection.
On the other hand, in the inner cam type fuel injection pump, in order to control an injection quantity of fuel injected from the fuel injection pump, the fuel quantity introduced in the pump chamber defined between the above-mentioned plungers is controlled conventionally by a throttle mechanism. The throttle mechanism has a throttle disposed in a passage which communicates the pump chamber with a supply source of fuel, and the size of an opening of the throttle can the pump chamber, i.e., the injection quantity of fuel varies in each injection stroke. Furthermore, since the viscosity of fuel changes in accordance with its temperature, it is difficult to control the injection be adjusted. Therefore, according to such a throttle mechanism, the quantity of fuel introduced into the pump chamber per stroke of the plungers, i.e., the injection quantity of fuel can be adjusted by changing the opening size of the throttle.
However, the quantity of fuel controlled by the above-mentioned throttle mechanism cannot be determined solely by the size of the opening of the throttle, but also depends on a differential pressure between the supply source of fuel and the pump chamber and the viscosity of the fuel. In the inner cam type fuel injection pump, the differential pressure between the supply source of fuel and the pump chamber can be determined by a supply pressure of fuel during introduction of fuel and a residual pressure in the pump chamber immediately after the pressurized fuel is delivered from the pump chamber. However, although the supply pressure of fuel can be easily maintained, the residual pressure of fuel cannot. Therefore, in the inner cam type fuel injection pump, since the differential pressure of fuel between the supply source of fuel and the pump chamber cannot be always maintained during introduction of fuel, an introduction quantity of fuel to the pump chamber, i.e., the injection quantity of fuel varies in each injection stroke. Furthermore, since the viscosity of fuel changes in accordance with its temperature, it is difficult to control the injection quantity of fuel with high accuracy in a inner cam type fuel injection pump.